Succinic acid is industrially used extensively as a food additive, starting material for medicines, starting material for resins, etc. Hitherto, succinic acid has been obtained by reducing petroleum-derived maleic anhydride or maleic acid. Recently, however, it has been reported that succinic acid is produced, in a high carbon yield, from a wide range of biomass-derived starting materials by a fermentation operation using microorganisms. Especially when succinic acid is to be used as a starting material for polymers, high-purity succinic acid is required for the purposes of maintaining the degrees of polymerization of the polymers, preventing the polymers from taking a color, etc.
Known as a process for producing succinic acid is, for example, a process in which maleic anhydride or maleic acid each derived from a petrochemical starting material is reduced into succinic acid in an aqueous solution and the resultant aqueous solution is cooled to cause crystallization and thereby obtain the succinic acid as crystal particles (for example, patent document 1). However, patent document 1 includes no statement concerning any method for crystallizing succinic acid.
Known as processes for producing succinic acid by fermentation are, for example, a process in which the calcium salt of succinic acid is decomposed with sulfuric acid (for example, patent document 2), a process in which an ion-exchange resin is used (for example, patent documents 3 and 4), a process in which ammonium hydrogen sulfate and/or sulfuric acid is added to the ammonium salt of succinic acid to yield succinic acid and ammonium sulfate (for example, patent document 5), and a process in which electrodialysis is used (for example, patent document 6). However, none of patent documents 2 to 6 includes a statement concerning a detailed crystallization method for recovering crystals of succinic acid.
With respect to crystallization operations, known techniques include a method in which the particle diameter and particle size distribution of succinic acid are regulated by regulating the stirring conditions and the degree of supersaturation (for example, patent document 7) and a method in which the particle diameter and particle size distribution of succinic acid are regulated by using seed crystals (for example, non-patent document 1). However, these methods each relate to the regulation of particle diameter and particle size distribution, and there is no statement therein concerning the purity of succinic acid.
Meanwhile, it is important, for the recovery of succinic acid through crystallization, to regulate the particle diameter and particle size distribution of the crystals, and various attempts have been made (for example, patent documents 8 and 9).
For example, patent document 8 describes a technique for obtaining purified succinic acid from a succinic-acid-containing fermentation broth produced by a fermentation method, the technique including adding an inorganic acid to the fermentation broth, from which cells of the microorganism have been removed beforehand, to adjust the pH of the fermentation broth to 4.0 or less and thereby temporarily precipitate the succinic acid, subsequently heating the liquid to partly dissolve and melt the succinic acid, thereafter cooling the mixture with stirring to precipitate crystals, and separating the crystals [from page (2), left upper column, line 14 to the same page, right upper column, line 3]. It is important that the fermentation broth which has undergone the acid precipitation treatment should be subjected to a heat treatment as described above. In case where the heat treatment is omitted, the crystallization results in flaky and fine crystals. In contrast, when the heat treatment is conducted, granular and large crystals are obtained by the crystallization [page (3), left upper column, lines 6-9].
Patent document 9 includes a statement to the effect that in a method for crystallizing succinic acid from a succinic-acid-containing solution, crystals having a large particle diameter and a narrow particle diameter distribution can be obtained by regulating the required-stirring-power ratio (Pv) to 70-350 W/m3 and the nondimensional degree of supersaturation (Sc) to 1-1.3 (claim 1 and paragraph 0009).
In patent document 8 and patent document 9, crystallization of succinic acid is conducted by a batch treatment.